Exhaust gas is generated during the combustion of fuels such as natural gas, gasoline, diesel fuel, fuel oil or coal. When the combustion occurs in a chamber such as an engine or furnace, the resulting exhaust gas is typically treated prior to being discharged into the atmosphere through an exhaust pipe, flue gas stack or the like. Although the largest portion of exhaust gas consists of nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2), which are considered to be innocuous compounds, untreated exhaust gas also contains relatively low amounts of undesirable noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) (e.g., nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O)) from excessive combustion temperatures, and particulate matter (insoluble carbon soot particles. In addition small amounts of liquid hydrocarbons (e.g. lube oil and unburned fuel), and various other organic compounds may be present. The present invention relates to exhaust gas emitted from diesel engines which typically operate using a high air-to-fuel ratio (i.e., very lean conditions). Such lean burn conditions often result in an exhaust gas with relatively high emissions of particulate matter and NOx—two components which have proved to be difficult to efficiently convert into more benign substances.
Diesel engines are often equipped with exhaust systems that include one or more catalytic components which work separately, or in combination, to treat exhaust gas prior to its emission into the atmosphere. For example, it is known that NOx can be converted into elemental nitrogen, N2, and water by reacting NOx in the exhaust gas with NH3 in the presence of certain supported catalysts via a process commonly referred to as selective catalytic reduction (SCR). Known SCR catalysts include vanadium (V2O5) carried by a mixture of ceria (CeO2) and alumina (Al2O3) on a support (see EP 0 246 859), or V2O5/WO3 supported on TiO2 (see WO 99/39809). Other SCR catalysts have also been proposed such as mixed metal oxides such as Fe—W/CeZrO2 (WO 2009/001131) and aluminosilicate and silicoaluminophosphate molecular sieves loaded with an extra-framework metal such as Cu:SAPO-34 (See is there is patent US 2010/0290963). NOx treatment involves the gas phase transformation of NOx into nitrogen gas (N2) and water vapor (H2O).
Unlike NOx treatment, soot remediation in an exhaust gas typically involves mechanical filtration. For example, soot emissions can be reduced by passing a soot-containing exhaust gas through a diesel particulate filter (DPF), such as a cordierite wall-flow filter (check if patent US 2010/0170230). When soot-containing exhaust gas is passed through a filter, the soot containing particles are removed from the gas and become placed on or in the filter. While this process can be effective in removal of the particles, the accumulation of soot particles on or in the filter can cause an undesirable increase in the back pressure across the filter. This increase in back pressure typically leads to decreased engine performance and efficiency. The accumulated carbon-based soot can be removed from the filter by regenerating the filter, which is typically accomplished by periodically combusting the soot. One such combustion technique involves catalytic oxidation of the soot at low temperatures via a soot oxidation catalyst incorporated onto the filter (i.e., a catalytic soot filter (CSF)) (U.S. Pat. No. 4,902,487).
Conventional exhaust systems comprise separate components for NOx treatment (SCR) and soot treatment (CSF). Yet it is often desirable to design individual exhaust components to perform more than one function in order to reduce the total space required by the exhaust system, reduce costs, etc. For example, applying an SCR catalyst to a filter substrate (SCRF) serves to reduce the overall size of an exhaust treatment system by allowing one substrate to serve two functions, namely catalytic conversion of NOx by the SCR catalyst and removal of soot by the filter. For example, U.S. Pat. Pub. 2010/0180580 discloses an SCR catalyst can be applied to a wall-flow DPF. However, removal of the soot oxidation catalyst in an SCRF requires the soot accumulated at the filter surface to be burned at very high temperatures. Accordingly, there remains a need for an efficient system for treating soot and NOx in lean burn exhaust gas. This invention satisfies that need among others.